1. Field of the Invention
The present invention relates generally to clamping circuits, more specifically, the present invention relates to a clamping circuit for power converters.
2. Description of the Related Art
FIG. 1 shows a primary-side controlled power converter which includes a transformer 10, a power transistor 20, a controller 100, rectifiers 40 and 50, capacitors 45 and 55, and a voltage divider. The controller 100 has an input terminal VS coupled to an auxiliary winding NA of the transformer 10. The voltage divider formed by a resistor 30 and a resistor 35 is connected in parallel with the auxiliary winding NA of the transformer 10. A reflected signal VNA generated by the auxiliary winding NA is attenuated by the voltage divider to generate a detection signal VS at a joint of the resistors 30 and 35. The detection signal VS is supplied to the input terminal VS of the controller 100.
The controller 100 further generates a switching signal VG coupled to control the power transistor 20 for switching a primary winding NP of the transformer 10. A secondary winding NS of the transformer 10 generates an output voltage VO via the rectifier 40 and the capacitor 45 at an output of the primary-side controlled power converter. The reflected signal VNA further charges the capacitor 55 via the rectifier 50 to supply a supply voltage VCC to a supply terminal VCC of the controller 100. The detail descriptions of the primary-side controlled power converter and the operation of detecting a detection signal of the transformer can be found in prior arts, such as U.S. Pat. No. 7,016,204 titled “Close-loop PWM Controller for Primary-side Controlled Power Converters”; U.S. Pat. No. 7,352,595 titled “Primary-side Controlled Switching Regulator”; and U.S. Pat. No. 7,486,528 titled”.
FIG. 4 shows the waveforms of the switching signal VG and the reflected signal VNA of the auxiliary winding NA of the transformer 10. A leakage inductance of the transformer 10 and a parasitic capacitance of the power transistor 20 cause an oscillating energy with an oscillation period TS1 at a leading edge of the reflected signal VNA when the switching signal VG is disabled. As this oscillation persists, the detection signal Vs cannot be accurately detected, which will slow down the regulation, especially for a high switching frequency transformer which is applied in the primary-side controlled power converter.
To achieve high-speed detection of the detection signal Vs, the oscillating period TS1 of the reflected signal VNA should be shortened to improve the regulation of the primary-side controlled power converter.